1. Field of the Invention
The present invention relates generally to a wireless communication system using an orthogonal frequency division multiplexing scheme, and more particularly to a system and a method for dynamically allocating a channel according to channel states.
2. Description of the Related Art
Currently, a 3rd generation (3G) mobile communication system is advancing to a 4th generation (4G) mobile telecommunication system. In the 4th generation (4G) communication system, which is a next generation communication system, a large amount of research is actively being carried out in order to provide users with various qualities of service (QoS) and a data transmission rate of about 100 Mbps.
A wireless local area network (LAN) system and a wireless metropolitan area network (MAN) system generally support a data transmission rate of 20 to 50 Mbps. Therefore, more specifically, a large amount of research is being conducted in the 4G communication system for developing a communication system providing a superior QoS and mobility through the wireless LAN system and the wireless MAN, in order to provide relatively high data transmission rate.
Because the wireless MAN system has a wide coverage area and supports a high data transmission rate, the wireless MAN communication system is suitable for a high-speed communication service. However, because the wireless MAN communication system does not consider mobility of a user, that is, a subscriber station (SS), a handover, which is required when the SS is moved, is not considered in the wireless MAN communication system.
An IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system communicates by performing a ranging operation between an SS and a base station (BS).
FIG. 1 is a block diagram schematically illustrating a conventional broadband wireless access communication system utilizing an orthogonal frequency division multiplexing (OFDM) scheme and an orthogonal frequency division multiple access (OFDMA) scheme. More specifically, FIG. 1 is a schematic view illustrating a structure of the IEEE 802.16a communication system.
However, prior to describing FIG. 1, the wireless MAN system is a broadband wireless access (BWA) communication system and has a wider coverage area and a higher data transmission rate as compared with those of a wireless LAN system. The IEEE 802.16a communication system is a system utilizing the OFDM scheme and the OFDMA scheme in order to provide a broadband transmission network for a physical channel of the wireless MAN communication system. That is, the IEEE 802.16a communication system is the broadband wireless access communication system utilizing the OFDM/OFDMA schemes. Because the IEEE 802.16a communication system uses the OFDM/OFDMA schemes in the wireless MAN system, physical channel signals can be transmitted through a plurality of sub-carriers, and a high-speed data transmission is possible.
An IEEE 802.16e communication system is a system reflecting mobility of the SS in addition to the IEEE 802.16a communication system. That is, both the IEEE 802.16a communication system and the IEEE 802.16e communication system are broadband wireless access communication systems utilizing the OFDM/OFDMA schemes, but the IEEE 802.16e communication system considers the mobility of a SS. Hereinafter, for the purpose of description, the IEEE 802.16a communication system will be described by way of example.
Referring to FIG. 1, the IEEE 802.16a communication system has a single cell structure and includes a base station (BS) 100 and a plurality of subscriber stations (SSs) 110 to 130, which are managed by the BS 100. The BS 100 transmits/receives signals to/from the SSs 110 to 130 using the OFDM/OFDMA schemes.
4G mobile telecommunication systems have been standardized in an attempt to provide a convergence of services and an efficient combination of a wired communication network and a wireless communication network, beyond simple wireless communication services provided by prior generation mobile telecommunication systems. Accordingly, there has been a request for a technique enabling the wireless communication network to transmit data having a large capacity, which is nearly equivalent to the capacity of data that can be transmitted by the wired communication network.
Therefore, in the 4G mobile telecommunication systems, the OFDM scheme has been actively studied as a useful scheme for transmitting data at a high speed through wired/wireless channels. The OFDM scheme uses a scheme for transferring data using multi-carriers. That is, the OFDM scheme is a kind of a Multi-Carrier Modulation (MCM) scheme for converting serially input symbols into parallel symbols, modulating the parallel symbols into multiple sub-carriers having mutual orthogonality, that is, multiple sub-carrier channels, and transmitting the multiple sub-carrier channels.
The OFDM scheme is similar to a conventional frequency division multiplexing (FDM) scheme. However, the OFDM scheme has a characteristic that multiple sub-carriers are transmitted while maintaining orthogonality to each other. As a result, the OFDM scheme is capable of obtaining the best transmission efficiency when data is transmitted at a high speed. Also, the OFDM scheme has superior frequency usage efficiency and a characteristic resistant to multi-path fading. Additionally, the OFDM scheme efficiently utilizes frequency by utilizing frequency spectrums that overlap each other. Further, the OFDM scheme is a highly resistant to frequency selective fading, multi-path padding, and impulse noises, can reduce Inter Symbol Interference (ISI) between symbols by utilizing guard intervals, and enables an equalizer to have a simple hardware structure.
Multiple mobile stations and a base station positioned within one cell have to share resources with each other in order to increase channel utilization in the OFDM system. One of the sharable resources in the OFDM system is a sub-carrier, which is channelized by the base station. Optimum channel utilization can be secured according to a method in which the base station allocates the sub-carriers to the mobile stations within the cell.
Methods for allocating the sub-carriers include a static allocation method and a dynamic allocation method. The static allocation method includes a Static Sub-carrier Assignment (SSA) scheme, a Pseudo Static Assignment scheme (PSA), and a Simple Rotating Sub-carrier Space Assignment (Simple RSSA) scheme. The dynamic allocation method includes a Fast Dynamic Channel Allocation (Fast DCA) scheme.
The SSA scheme is the simplest scheduling method in which a predetermined number of sub-carriers are statically assigned to mobile stations. Particularly, in the SSA scheme, all the selected sub-carriers are assigned to mobile stations regardless of channel states. Although the SSA scheme ensures equality when channels are allocated to mobile stations, the SSA scheme cannot ensure quality of sub-carriers allocated to mobile stations.
In the PSA scheme, sub-carriers statically allocated to mobile stations can be reassigned by changing the sub-carriers between the mobile stations. More specifically, the PSA scheme changes sub-carriers, which can prevent channel quality of mobile stations from being degraded and assigns sub-carriers in a good condition to mobile stations within a range of allocable sub-carriers, thereby raising transmission efficiency.
The Simple RSSA scheme allocates sub-carriers on the basis of priorities. For example, the priorities can be arranged according to a quality of service (QoS). More specifically, through the simple RSSA scheme, although the same number of sub-carriers are allocated to mobile stations, a sub-carrier with good condition is allocated to a mobile station with a highest priority first, and then, sub-carriers with relatively low quality are allocated as priority is lowered. The simple RSSA scheme can provide good QoS. However, the RSSA scheme causes differentiated allocation between mobile stations when sub-carries are allocated the mobile stations.
The Fast DCA scheme form among the dynamic assignment methods minimizes intra-cell interference and inter-cell interference and allocates the optimal channel to a mobile station on the basis of a channel condition.
FIG. 2 illustrates a time relation during a dynamic channel allocation, depending on a decision of an access point in a conventional OFDM mobile telecommunication system. Referring to FIG. 2, a mobile station 200 periodically transfers channel quality information (CQI) to an access point 220 to which the mobile station 200 belongs in step 202. For example, the CQI may include a signal-to-noise ratio (SNR). The access point 220 selects a sub-channel to be allocated to the mobile station 200 by means of the CQI carried according to frame cells. The sub-channel is selected by selecting optimal frame cells, then, by selecting idle sub-channels from among sub-channels corresponding to the optimal frame cells. The access point 220, which has selected the sub-channel, allocates the sub-channel of the selected frame cells to the mobile station 200 such that the mobile station 200 can utilize the sub-channel in step 222. The mobile station 200, which has received information about the sub-channel allocation, transmits signals through the allocated sub-channel.
As illustrated in FIG. 2, the access point 220 performs an overall process of analyzing CQI according to frame cells transmitted from the mobile station 200, selecting an optimal channel to be utilized by the mobile station 200, and allocating the optimal channel to the mobile station 200. As described above, because the access point 220 performs the overall process, it is possible to reduce back-haul delay time consumed in a network end.
However, when the mobile station 200 performs a handover, the access point 220 must transfer information thereof and CQI of the mobile station 200 to an access router 240 in order to progress a handover process in step 224. The access router 240 enables the mobile station 200 to perform the handover using CQI transmitted from the access point 220 in step 244, and transfer handover process information to the access point 220 in step 226. The access point 220 performs the handover according to the handover process information received from the access router 240 in step 230.
As described above, when the handover of the mobile station 200 occurs, the access point 220 must receive the handover process information for the mobile station 200 from the access router 240, and then, transfer the handover process information to the mobile station 200. Accordingly, a delay occurs, which is as long as total process time for handover illustrated by reference numeral 206 of FIG. 2, and it is impossible to rapidly support the handover of the mobile station because of the delay. In addition, when packets are transmitted from the access point 220 to the access router 240 in order to perform the handover, the packets may be repeatedly transmitted or may be lost. Therefore, the packets must be transmitted with sequence numbers in order to prevent the packets from being lost.
FIG. 3 illustrates a process of dynamically allocating channels depending on a decision of an access router in a conventional OFDM mobile telecommunication system according to a passage of time. Referring to FIG. 3, a mobile station 300 periodically transmits CQI to an access point 320 to which the mobile station 300 belongs according to frame cells in step 302. The access router 340 receives the CQI from the access point 320 in step 322, and arranges frame cells in order of superior quality based on the CQI during an access router process time. In selection of sub-channels of the frame cells by the access router 340, a frame cell having a higher quality is assigned prior to other frame cells or frame cells having a quality higher than a predetermined standard are first assigned from among the frame cells having superior CQI.
As described above, information about frame cells selected is transmitted to the access point 320. More specifically, the mobile station 300 transmits CQI for ten frame cells to the access point 320. The access router 340 receives the CQI from the access point 320 and selects five frame cells from among the ten frame cells in order of superior quality. It is assumed that the selected five frame cells have sequence numbers of ‘4’, ‘2’, ‘7’, ‘9’, and ‘1’, respectively, and each frame cell has three sub-channels of ‘A’, ‘B’, and ‘C’. It is noted that the number of the sub-channels is a changeable variable when each frame is designed.
The access router 340 searches sub-channels of ‘A’, ‘B’, and ‘C’ of the fourth frame cell with the best quality from among the selected five frame cells, and selects an idle sub-channel from among the three sub-channels. If all sub-channels of the fourth frame cell are allocated to other mobile stations, the access router 340 searches sub-channels of ‘A’, ‘B’, and ‘C’ of the second frame with the next superior quality. The above search process is repeated up to a first frame until a sub-channel is selected. If there is no sub-channel selected even though the search process is performed up to the first frame, the search process is carried out with respect to sub-channels of remaining five frame cells excluding the above five frame cells. The access point 320, which has received information about a sub-channel of a frame cell selected through the search process, allocates the sub-channel to the mobile station 300. The access router 340 updates information about sub-channels and frame cells thereof on the basis of the information on a frame cell and a sub-channel transferred to the access point 320 in order to consider the information when selecting a sub-channel of a frame cell to be allocated to another mobile station.
As described above, the access router 340 receives CQI according to frame cells transmitted from the mobile station 300 through the access point 320. The access router 340 arranges the frame cells in order of superior quality, performs a process regarding selection of a sub-channel of a frame cell to be allocated to the mobile station 300, and transfers information about the sub-channel to the access point 320. The access point 320 allocates the sub-channel of the frame cell to the mobile station 300.
Consequently, a time delay is created of the same length as that of the sum of a duration 342 in which the mobile station 300 transfers the CQI to the access point 320 and the access router 340 receives CQI, a duration 306 in which the access router 340 selects an assembly of sub-channels of frame cells on the basis of the received CQI about the frame cells and transfers the assembly to the access point 320, and a duration 308 in which the access point 320 allocates the received sub-channel of the frame cell to the mobile station 300. Accordingly, although the access router 340 allocates a sub-channel of a frame cell on the basis of initially-transferred CQI 302 to mobile station 300, the allocated sub-channel does not reflect a change of a channel condition according to the time delay. Therefore, the allocated sub-channel quality may not be optimized for the mobile station 300. Accordingly, a new dynamic channel allocating method is required that reflects a channel condition changing until the mobile station 300 is allocated a channel, after initially transferring CQI 302.